Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semi-conductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semi-conductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semi-conductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semi-conductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and inject holes or electrodes. For example, a p-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO level of the semi-conductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO level of the semi-conductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form thin film transistors. When an organic material is used as the semi-conductive material in such a device, it is known as an organic thin film transistor (OTFT). OTFTs may be manufactured by low cost, low temperature methods such as solution processing. Moreover, OTFTs are compatible with flexible plastic substrates, offering the prospect of large-scale manufacture of OTFTs on flexible substrates in a roll-to-roll process.
Various arrangements for organic thin film transistors are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semi-conductive material disposed therebetween in a channel region, a gate electrode disposed adjacent the semi-conductive material and a layer of insulating material disposed between the gate electrode and the semi-conductive material in the channel region.
An example of such an organic thin film transistor is shown in FIG. 1. The illustrated structure may be deposited on a substrate 1 and comprises source and drain electrodes 2, 4 which are spaced apart with a channel region located therebetween. An organic semiconductor (OSC) 8 is deposited in the channel region and may extend over at least a portion of the source and drain electrodes 2, 4. An insulating layer 10 of dielectric material is deposited over the organic semi-conductor 8 and may extend over at least a portion of the source and drain electrodes 2, 4. Finally, a gate electrode 12 is deposited over the insulating layer 10. The gate electrode 12 is located over the channel region and may extend over at least a portion of the source and drain electrodes 2, 4.
The structure described above is known as a top-gate organic thin film transistor as the gate is located on a top side of the device. If source/drain contacts are located under the OSC layer the device may be more completely described as a top gate, bottom contact device. Top gate, top contact devices are also possible, with the source/drain contacts located over the OSC.
Alternatively, it is also known to provide the gate on a bottom side of the device to form a so-called bottom-gate organic thin film transistor.
An example of such a bottom-gate organic thin film transistor is shown in FIG. 2. In order to show more clearly the relationship between the structures illustrated in FIGS. 1 and 2, like reference numerals have been used for corresponding parts. The bottom-gate structure illustrated in FIG. 2 comprises a gate electrode 12 deposited on a substrate 1 with an insulating layer 10 of dielectric material deposited thereover. Source and drain electrodes 2, 4 are deposited over the insulating layer 10 of dielectric material. The source and drain electrodes 2, 4 are spaced apart with a channel region located therebetween over the gate electrode. An organic semiconductor (OSC) 8 is deposited in the channel region and may extend over at least a portion of the source and drain electrodes 2, 4.
If the source/drain contacts are located under the OSC layer, the device may be more completely described as a bottom gate, bottom contact device. Bottom gate, top contact devices are also possible, with the source/drain contacts located over the OSC.
The conductivity of the channel can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage.
The dielectric layer comprises a dielectric material selected from insulating materials having a high resistivity. The dielectric constant, k, of the dielectric is typically around 2-3 although materials with a high value of k are in principle desirable because the capacitance that is achievable for an OTFT is directly proportional to k, and the drain current ID is directly proportional to the capacitance. However, research has also shown that in many cases the properties of the OTFT is improved with lower k dielectric materials, as is the case with the fluorinated materials discussed below.
The drain current that is achievable for an organic thin film transistor is inversely proportional to the thickness of the dielectric in the active region of the device (channel between source and drain electrodes). Thus, in order to achieve high drain currents with low operational voltages, organic thin film transistors must have thin dielectric layers in the channel region.
It is apparent from the above that the dielectric in an organic thin film transistor, and the interface it forms with the OSC, is an important factor in determining the operating characteristics of the organic thin film transistor. As such, various materials and structures for the dielectric have been proposed in the prior art.
U.S. Pat. No. 6,265,243, which was published in 2001, discloses an OTFT in which the dielectric is surface-treated with a fluorinated organic material such as a fluorinated silane. Suitable materials for the dielectric are given as silicon dioxide, polyimide, and polyvinylphenol (PVP). It is also disclosed that as an alternative to treating the dielectric surface with fluorinated organic material such as fluorinated silane, the dielectric material may be replaced with a dielectric polymeric material rich in fluoroalkyl chains. No examples appear to be given for this alternative dielectric polymeric material.
Applied Physics Letters, vol. 85, no. 12, p 2283 (2004) describes a bilayer dielectric. This document describes using a bilayer comprising PVP and polyvinyl acetate as the two layers.
U.S. Pat. No. 7,279,777, which was published in 2007 and refers to the previously mentioned document U.S. Pat. No. 6,265,243 in its background section, discloses substantially non-fluorinated cyano-functional polymers for the dielectric layer which preferably include a crosslinkable group. Various non-fluorinated styrene-containing surface modifying polymers are also disclosed for use with the non-fluorinated cyano-functional polymers. It is described that much higher mobilities are achieved than for the fluorinated dielectrics disclosed in U.S. Pat. No. 6,265,243.
It is one aim of embodiments of the present invention to provide a solution to one or more of the problems discussed above.